Looking back I think any replacement product needs to be 100% compatible and after x months clients are forced over.
I actually think this is a ploy to get their users to convert. 37signals has a strong "we let our users outgrow us" stance, so what I read between the lines is that 37signals sees no hassle continuing to support the old system (it's massively profitable). Dually it gives a better justification for an up-sell to get people on to the new system.
I think this strategy is brilliant.